BRECKNOCKSHIRE 163 



the second highest summit, Van Voel, reaching 2632 feet. 

 Most of these mountains have rounded summits which are 

 smooth and covered with grassy or sedgy vegetation, but 

 many of them have some craggy slopes or precipices on their 

 northern faces. 



Almost the whole of this region is of the Old Red Sand- 

 stone formation, which here consists of nearly horizontal 

 strata with a moderate dip to the south ; and the whole of 

 the very numerous valleys with generally smooth and 

 gradually sloping sides which everywhere intersect it, must 

 be all due to sub-aerial denudation — that is, to rain, frost, and 

 snow — the debris due to which is carried away by the brooks 

 and rivers. The geologist looks upon the rounded summits of 

 these mountains as indications of an extensive gently undu- 

 lating plateau, which had been slowly raised above the surface 

 of the lakes or inland seas in which they had been deposited, 

 and subjected to so little disturbance that the strata remain 

 in a nearly horizontal position. When from the summit of 

 any of these higher mountains we look over the wide parallel 

 or radiating valleys with the rounded grassy ridges, and con- 

 sider that the whole of the material that once filled all these 

 valleys to the level of the mountain-top has been washed away 

 day by day and year by year, by the very same agencies that 

 after heavy rain now render turbid every brooklet, stream, 

 and river, usually so clear and limpid, we obtain an excellent 

 illustration of how nature works in moulding the earth's 

 surface by a process so slow as to be to us almost imper- 

 ceptible. 



This process of denudation is rendered especially clear to 

 us by the singular formation of the twin summits of the Brecon 

 Beacons. Here we are able, as it were, to catch nature at 

 work. Owing to the rare occurrence of a nearly equal rate 

 of denudation in four or five directions around this highest 

 part of the original plateau, we have remaining for our 

 inspection two little triangular patches of the original peat- 

 covered surface joined together by the narrow saddle, as 

 shown in the sketches opposite, showing a plan of the summits 

 and a section through them to explain how accurately the two 



